Silicon electro-optic modulators play an increasing role in the field of optic communication due to its process compatibility with CMOS technology. Standard silicon electro-optic modulators employ MOS capacitors or reverse biased PN diodes to achieve a large modulation bandwidth even up to 30 GHz. Silicon electro-optic modulators with MOS capacitors have the merit of higher modulation efficiency at the expense of optic power compensating for the loss from the poly-silicon, which includes one part of the capacitors. Silicon electro-optic modulators with PN diodes have the advantage of lower optic loss at the expense of modulation efficiency. The modulation depth, i.e., the total phase change of the modulator, is proportional to the modulation efficiency and the length of the modulator. Thus, it generally takes much longer for modulators with PN diodes than those with MOS capacitors to obtain the same modulation depth. For such a longer PN type modulator, a properly designed microwave traveling electrode such as a coplanar waveguide or a coplanar strip line is required for high speed modulation. It is preferable that the length of the electrodes is kept as short as possible for high speed modulation. However, there exists a dilemma for the PN type modulator to have the conflicting requirements of a shorter device for high speed modulation and a longer device for big modulation depth.
The modulation efficiency of a modulator is proportional to the capacitance per unit length of the modulator. Higher capacitance per unit length means higher microwave loss and lower modulation speed. A trade-off between the modulation efficiency and modulation speed must be made. Also, higher capacitance per unit length gives rise to lower microwave impedance in the order of 20˜30 Ohm. It makes the impedance matching to the modulator driver difficult, and a specially designed driver with output impedance much lower than standard 50 Ohm is required. In the meantime, the low microwave impedance of the modulator and driver increases the RF power consumption of the transmitter employing the modulator.